This invention relates to apparatus for indicating the phase of the color subcarrier signal of a color television signal and, more particularly, to such apparatus which is capable of producing a line identification signal to identify whether the horizontal line interval of the color television signal is odd or even, in accordance with the detected phase polarity of the color subcarrier signal.
In conventional television systems, such as the NTSC system, the chrominance subcarrier signal is modulated with color information. The phase of this chrominance subcarrier signal is shifted from line-to-line. For example, and with reference to the NTSC system, the phase of the chrominance subcarrier signal is inverted at each horizontal line interval. Thus, for odd-numbered lines, the phase of the chrominance subcarrier signal may be considered to be "positive", and then, for the next-following even-numbered lines, the phase of this chrominance subcarrier signal is inverted and may be considered to be "negative". That is, during odd-numbered lines, the chrominance subcarrier signal may be thought of as exhibiting 0.degree. phase, and during even-numbered lines, the chrominance subcarrier signal may be thought of as exhibiting 180.degree. phase.
In various signal processing techniques, such as in video recording/reproducing applications, time base error correction applications, and the like, it is important to identify the phase of the chrominance subcarrier signal during each line interval. It has been proposed that a line identification signal be produced, indicating whether the line of color television signals is odd (and, thus, the chrominance subcarrier signal exhibits 0.degree. phase) or is even (and, thus, the chrominance subcarrier signal exhibits 180.degree. phase). This odd/even identification of the line of video signals thus identifies the relative phase of the chrominance subcarrier signal.
Typically, in a composite color television signal, the burst signal, which is provided on the back porch of the horizontal synchronizing signal, is phase- and frequency-locked to the chrominance subcarrier signal in that line interval. The line identification signal thus may be obtained by detecting the relative phase of the burst signal. It has been thought that this phase detection can be achieved by using the horizontal synchronizing signal as a reference point, and then sensing the phase of the burst signal at a predetermined time from this reference point. For example, if the burst signal is shaped to be substantially rectangular, a first phase (e.g. 0.degree., or positive phase) is detected if the burst signal exhibits a positive, or relatively higher level when sensed, and a second phase is detected when the burst signal exhibits a negative, or relatively lower level, when sensed. The detected first phase thus is representative of an odd line, and the detected second phase is representative of an even line.
However, in the foregoing, the phase of the burst signal is sensed, or sampled, at a predetermined time following the occurrence of the horizontal synchronizing signal. For example, the burst signal may be sampled at a predetermined time following the trailing edge transition of the horizontal synchronizing signal. Unfortunately, phase shifts may occur between the horizontal synchronizing signal and the burst signal, due to various factors in the transmission or reception of the color television signal. Furthermore, changes in the response characteristics of the phase-detecting circuitry may occur due to temperature drift, thus resulting in an erroneous line identification. As a consequence of the foregoing, it has become difficult to obtain stable, accurate phase detection of the burst signal and, thus, it has become difficult to produce accurate line identification signals.